1. Field
The present invention relates to hot water heater control devices and in particular to an apparatus and method for controlled heating of water during off peak load hours.
2. Description of Prior Art
U.S. Pat. No. 2,238,624 to Clark illustrates an off peak hour water heater control. This control is designed to turn on a hot water heater at the proper time to heat the water to a desired temperature by a specified hour when the heater is turned off. Both the time and temperature are measured continuously so that the heater will be turned on when the water falls to the "critical temperature" at that moment. Basically the water is at its "critical temperature" when the length of time required to heat the water at full heater power, from its present temperature to the desired temperature, equals or exceeds the length of time remaining until the specified cut off hour. For example, assuming a desired temperature of 180.degree. by 8:00 A.M. and further assuming a hot water heater that heats the water 10.degree. per hour, at full power some critical temperatures are as follows:
100.degree. at 12:00 A.M. PA0 120.degree. at 2:00 A.M. PA0 140.degree. at 4:00 A.M. PA0 160.degree. at 6:00 A.M.
Thus, the heater is turned on at 12:00 A.M. if the water is 100.degree. or less.
The control can be manually adjusted to compensate for different full power heating rates of different hot water heaters.
Although Clark attempts to heat the water to a desired peak temperature by a predetermined time, his device is not designed to adjust the heating element to only a portion of its maximum power; rather, it always operates at full power. Thus, Clark acts only to turn on the hot water heater at the appropriate starting time and turn off the heater at the specified hour. In contrast to Clark, the present invention adjusts the power input to the heater in order to attempt to heat the water using less than the maximum heater power. By adjusting the power of the heater, the water can be heated over a predetermined period of time while automatically compensating for the characteristics of the heater, the starting temperature of the water and hot water use during the time interval.
There are various other timing devices on the market for turning on and off hot water heaters or related appliances. However, none of these timing devices take advantage of the potential load spreading afforded by adjusting the input to the heater in order to raise the temperature of the water over a predetermined time interval. Additionally, none of these can automatically compensate for hot water use during the time interval.